A transportation concept or implementation that is touted as futuristic or innovative, but in execution turns out to be more expensive and less feasible than traditional means, outweighing their usefulness. Examples include but are not limited to Monorails (most often cited example) Personal Rapid Transit, Maglev, etc, versus traditional means generally referring to rail, subway, and road infrastructure

The term generally carries a negative connotation, in that the solutions may be seen as an expensive 'white elephant' in the truest sense: more difficult and more expensive to maintain and add to without any perceived benefit other than looking cool

References:

Wiktionary, gadgetbahn
Wikipedia, White Elephant disambiguation

A term used to reference cannabis/marijuana/weed.

Any mode of transport that is "futuristic" or "innovative" in theory, but ends up being either infeasible, impractical, or just a steaming pile of shit in practice.
Gadgetbahn systems are often outperformed by the most basic things, such as buses.